1. Field of the Invention
The present invention relates generally to a control system for an automotive vehicle having a four wheel driving power train layout. More specifically, the invention relates to a driving power distribution control system for a four wheel drive vehicle which assures precise control of power distribution corresponding to vehicle driving condition and is successful in avoiding influence any variation in the wheel speed.
2. Description of the Background Art
Japanese Patent First (unexamined) Publication (Tokkai) Showa 62-31529 which has been owned by the common owner to the present invention, discloses a power distribution control system for a four wheel drive automotive vehicle. The disclosed system includes wheel speed sensors respectively monitoring rotation speeds of a primary and a subsidiary driving wheels, a vehicle speed sensor for monitoring vehicle traveling speed, and a steering angle sensor for monitoring steering angular displacement. The control system detects vehicle driving condition on the basis of primary and subsidiary driving wheel speed, vehicle speed, steering angular displacement and so forth for adjusting driving power distribution ratio for the primary and the subsidiary driving wheels. With such prior proposed driving power distribution control systems, power distribution for the primary and subsidiary driving wheels can be well adjusted so as to provide satisfactorily high vehicle driving performance and stability.
On the other hand, in the modern and advanced automotive vehicles, various electronic or electric control systems are facilitated for respectively independent or associated control operations for various components of the vehicle. A typical and trendy control to be facilitated for the automotive vehicle is wheel slippage dependent acceleration and/or deceleration control, such as traction control, anti-skid brake control and so forth. In such wheel slippage dependent control, wheel speed is controlled so as to maintain optimal level of road/tire traction for vehicular acceleration and deceleration. Therefore, when wheel slippage dependent control is active, wheel speed can cyclically or periodically vary irrespective of a vehicle speed.
When such wheel slippage dependent control is coupled with the power distribution control, a difficulty is therefore encountered in obtaining vehicle speed data. Namely, it is a typical way for obtaining vehicle speed data on the basis of the wheel speed data since the wheel speed data normally reflects vehicle traveling speed. Further, the vehicle speed data is typically monitored by monitoring the average rotation speed of rear wheels. In such a method of monitoring the vehicle speed, the vehicle speed data obtained through the average rotation speed of the rear wheels may be inaccurate when the wheel speed is adjusted for better traction by the wheel slippage dependent control.